On the RMN site it states:
How long will an in-store cash back offer stay linked?
As long as the offer is valid (this varies by offer). Please note that you will need to wait 1-7 days, or until the first purchase is automatically awarded to your RetailMeNot wallet, before you can reuse the offer. Once you’ve used the offer, if you wish to reuse it simply click on the offer again. If the charge is showing as “pending” with your bank then it has not cleared.
My thoughts:
If you can see it and activate it, then maybe reusing it will work? I don't know. I'm personally not willing to risk it, but if you were planning to make a purchase from that merchant again anyway, I don't suppose it could hurt to link it. Although RMN has until mid-May to approve a purchase made now, it will most likely go through much faster than that.